JP4574861B2 - Gain flattening in fiber amplifiers. - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
Field of Invention
The present invention relates generally to optical amplifiers, and more particularly to an apparatus and method for amplifying optical signals of different wavelengths so that the optical signals obtain substantially the same gain.
[0002]
Explanation of related technology
Commercially available erbium-doped fiber amplifiers (EDFAs) currently have gains over a wide optical bandwidth (up to 50 nm for silica-based fibers). Over this bandwidth, the gain can be highly dependent on the wavelength of the input signal. However, in many applications, particularly long distance fiber communications, it is highly desirable to operate with wavelength independent gain. In order to take advantage of the vast fiber bandwidth, signals with different wavelengths within the gain bandwidth of the EDFA are simultaneously carried on the same fiber bus. If these signals gain different gains, they have different power at the output of the bus. This imbalance becomes more severe as the signal passes through each successive EDFA and can be significant for very long distances. For example, at the output end of a transoceanic bus involving a large number of EDFAs, a signal that obtains a lower gain per EDFA may carry only a few tenths of 1 dB of power to obtain a higher gain. For digital systems, the difference in signal power levels should not exceed 7 dB, otherwise lower power signals will be too noisy to use. EDFA gain flattening will eliminate this problem and produce an amplifier that can support significant optical bandwidth and hence higher data rates. Since the anticipated global demand for EDFA is very high, the development of methods to flatten amplifier gain while maintaining high power efficiency continues to be very important.
[0003]
During the past few years, several methods have been developed to produce EDFAs with as flat a gain as possible over the widest possible spectral range. The first method is to adjust both the fiber parameters (erbium concentration, index profile, core dopant nature and concentration) and pump parameters (power and wavelength). This method can produce a relatively flat (± 1-2 dB) gain, but only over a spectral region with a spectral width on the order of 10 nm, which is overly restrictive in most applications.
[0004]
Another method is to replace each of the EDFAs with a combination of two coupled fiber amplifiers, each of which has a gain dependence on different signal wavelengths. These dependencies are designed to compensate for each other and to produce fiber amplifier combinations that have gains that are substantially wavelength independent over a wide spectral range (eg, M. Yamada, M. Shimizu, Y. Ooiishi, “Er” of M. Horigushi, S. Sudou, and A. Shimizu³⁺Doped SiO₂-Al₂O_ThreeFiber and Er³⁺Flattening the gain spectrum of an erbium-doped fiber amplifier by connecting an Er³⁺-Doped SiO₂-Al₂O_Three Fiber and an Er³⁺-doped Multicomponent Fiber ")" Electron. Lett., Vol. 30, no. 21, pp. 1762-1765, October 1994). This is accomplished by using fibers with different hosts (eg, fluoride and silica fibers) and an EDFA combined with a Raman fiber amplifier.
[0005]
A third gain equalization method is to add a filter at the signal output end of the Er-doped fiber, which introduces losses in the part of the spectrum that exhibits higher gain. This strategy is demonstrated using filters made from standard blazed fiber gratings. (For example, R. Kashyap et al., “Wideband Gain Flattened Erbium Fiber Amplifier Using a Photosensitive Fiber Blazed Grating”) Electron. Lett , Vol. 29, pp. 154-156, 1993) This strategy is also demonstrated using filters from long-period fiber gratings (eg, AM Wengsarker ( AM Vengsarkar et al., “Long-Period Fiber-Grating-Based Gain Equalizers”, Opt. Lett., Vol. 21, pp.336-338, March 1996. See).
[0006]
The fourth method is gain clamping. In this measure, an EDFA is placed in an optical resonator to emit laser light. For laser cavities above a threshold at a given laser wavelength, the round trip gain is equal to the round trip loss (eg, Y. Tsao, J. Bryce and R. Minashan (Y Zhao, J. Bryce, R. Minasian, “Gain Clamped Erbium-doped Fiber Amplifiers-Modeling and Experiments” IEEE J. of Selected Topics in Quant. Electron., Vol. 3, no. 4, pp. 1008-1011, August 1997).
[0007]
In the gain clamping experiment of Cao et al., A specific wavelength λ₀Since the resonator is made by two fiber gratings that show high reflectivity only over a very narrow bandwidth in the vicinity (and very little at other wavelengths in the gain spectrum of the erbium-doped fiber), the laser emission Is this wavelength λ₀Only happens in λ₀Selection has a great influence on the spectral shape of the EDFA gain. Appropriate laser wavelength λ₀By selecting (1508 nm in their experiment), the gain spectrum can be relatively flat over a very wide range. Furthermore, λ₀The gain at is clamped to the value of the resonator loss for this wavelength for any pump power exceeding the threshold. As the gain spreads uniformly, the gain at other wavelengths also remains independent of the pump power (assuming the pump power is above the threshold).
[0008]
Another way to flatten the gain of the gain clamp EDFA is to rely on non-uniform spread of laser ions. Although specifically described herein as “laser ions”, this description is applicable to any particle that produces a laser via stimulated emission, such as ions, atoms, and molecules. In a purely uniformly spread laser medium, all ions exhibit the same absorption and emission spectra. When such a material is pumped below the laser threshold, the round trip gain is lower than the laser resonator round trip loss at all frequencies across the laser gain spectrum, as shown in FIG. The loss was expected to be independent of frequency across the gain spectral region. If pumped just above the threshold, this is the wavelength λ that satisfies the condition gain = loss.₁Oscillation starts at (see FIG. 1B). When the pump power is further increased (FIG. 1 (C)), the condition that gain = loss is λ₁At λ, ie λ₁The gain at is kept constant. This is understood from the physical point of view as follows. As the pump power increases, the population inversion increases, which produces stronger laser emission. While circulating through the fiber, this larger laser signal, via stimulated emission, reduces the inversion distribution enough to keep the gain equal to the loss. Furthermore, since the spread is uniform, all ions are λ₁Contributes equally to the gain at, thus the gain spectrum does not change. Naturally, the laser wavelength (λ₁) And the laser line width also remain the same (see FIG. 1C), ie they are independent of pump power. This is the principle of the above-described gain stabilization method.
[0009]
However, in a laser medium that spreads intensely and non-uniformly, not all of the ions exhibit the same absorption and emission spectra. One reason for this behavior is that not all of the physical locations where the laser ions are are the same. For example, in the case of an aluminum-doped silica-based host, the laser ions can be located next to silicon ions, oxygen ions, or aluminum ions. Laser ions present at the same location (eg, all laser ions present next to Si ions) exhibit the same absorption and emission spectra, ie they behave uniformly with respect to each other. In contrast, laser ions present at different positions exhibit different absorption and emission spectra, for example when one is located next to the Si ion and the other laser ion is located next to the Al ion, i.e. these Behave non-uniformly relative to each other. In the case of non-uniform spread, the laser medium can thus be considered as a collection of subsets of laser ions. Ions in a given subset behave uniformly, while ions in different subsets behave non-uniformly.
[0010]
When a non-uniformly spread material is pumped below the laser threshold, assuming a round trip loss independent of frequency across the gain spectrum region, the laser gain spectrum is The round trip gain is lower than the laser resonator round trip loss at all frequencies. If this material is pumped just above the threshold, this is primarily due to the wavelength λ satisfying the condition gain = loss.₁(See FIG. 2B, compare with FIG. 2A, which is below the threshold). This laser emission is mainly λ₁Involved in ion subsets exhibiting significant gains. As the pump power increases, laser emission at other wavelengths begins to appear, but the condition of gain = loss remains satisfied as shown in FIG. The laser medium again meets this condition by generating just enough laser power to exactly reduce the inversion distribution by the increase in inversion distribution due to the increase in pump power. Thus λ₁The gain at is “clamped” to the value of the loss. However, since the spread is non-uniform, λ₁The gain obtained from other ion subsets that peak at other wavelengths is λ₁Is not reduced so much to approach the loss value. Thus, as the pump power increases, these other wavelengths (eg, wavelength λ₂) Until the level of loss at that wavelength is reached, and the medium₂Start laser emission. At this point, the gain is λ₁And λ₂Is clamped at both. Generally, the gain curve is bell-shaped, so λ₂Is very λ₁(FIG. 2C). As more pump power enters the fiber (FIG. 2D), more wavelengths begin to laser. In practice, these discrete laser lines actually have a finite linewidth. Thus, if these discrete lines are close enough to each other, they merge with each other and this increase in laser lines ultimately results in a broadening of the laser linewidth. In summary, a non-uniformly spread laser medium tends to produce a broadened laser emission with increasing pump power. The laser linewidth is mainly in this manner and can be increased until it reaches the gain linewidth.
[0011]
In general, Er³⁺Laser transitions of triple ionized rare earth elements such as are spread by both homogeneous and non-uniform processes. The uniform mechanism broadens the line width of the transition between the Stark sublevels of erbium ions in the same manner as for all Er ions in the host. However, some non-uniform mechanisms result in changes in the Stark sublevel distribution that are not the same for all ions but depend on ion subsets.
[0012]
At room temperature, the 1.55 μm transition in Er-doped silica spreads mainly uniformly. However, cooling the material to a cryogenic temperature can produce a laser that oscillates over a relatively broad spectral range with constant gain and a constant gain (gain equals resonator loss). This effect has been used to produce a flat gain in an EDFA operated at 77 ° K. (For example, VL da Silva, V. Silberberg, JS Wang, EL Goldstein, VL da Silva, V. Silberberg, JS Wang, EL Goldstein) MJ Andrejco) “Automatic gain flattening in optical fiber amplifiers via clamping of inhomogeneous gain”, IEEE Phot.Tech Lett., VOl. 5 No. 4, pp. 412-14, April 1993), however, this approach is generally not practical in practice because the device requires the fiber to cool.
[0013]
SUMMARY OF THE INVENTION
The preferred embodiment of the present invention uses the non-uniform broadening of the erbium 1.55 μm transition to produce flat gain in an erbium-doped fiber amplifier without the need to cool the fiber to cryogenic temperatures. Gain broadening can be induced by pumping the fiber at the end of the absorption band of erbium ions, as opposed to existing erbium-doped fiber amplifiers (EDFAs) pumped near or at the center of the 980-nm absorption band. Is. Alternatively, the erbium-doped fiber may be pumped at multiple wavelengths simultaneously to excite multiple subsets of erbium ions and produce gain across the widest possible spectral region. For example,^FourI_15/2→^FourI_11/2For pumping to the transition, the pump wavelength can be uniformly dispersed or otherwise between about 970 nm and about 990 nm including a substantial portion of the absorption spectrum. The ideal spectral range of this pumping spectrum depends on the absorption spectrum of the particular erbium-doped fiber used, which itself depends on the co-dopant appearing in the core region of the fiber.
[0014]
One preferred embodiment of the present invention is an optical amplifier including an optical resonator for generating a clamped gain, the resonator including a gain medium having an absorption profile and a gain profile, the gain The profile is characterized at least in part by a non-uniform spread. The optical amplifier further includes an optical pump source that pumps the gain medium at at least one wavelength other than the peak of the absorption transition of the gain medium and flattens the gain using non-uniform broadening. In one preferred embodiment, the optical resonator is a ring resonator and the gain medium includes a doped fiber.
[0015]
Yet another preferred embodiment of the present invention is an optical amplifier including an optical resonator for generating a clamped gain, the resonator including a gain medium having an absorption profile and a gain profile, the gain The profile is characterized at least in part by a non-uniform spread. This embodiment further includes an optical pump source that pumps the gain medium at other than the peak of the absorption transition of the gain medium and alters the gain using non-uniform broadening, and further adjusts the loss to achieve the desired gain profile. It also includes a wavelength dependent loss element to produce.
[0016]
Yet another preferred embodiment of the present invention is a method for producing an optical amplifier having a substantially flat gain, the method introducing a pump signal into a gain medium having an absorption profile and a gain profile. The gain medium is in the resonator. The gain profile is characterized at least in part by a non-uniform spread, and the spectral output of the pump signal is selected to pump other than the peaks of the absorption profile to use the non-uniform spread of the gain medium. The method further includes injecting a plurality of optical signals of different wavelengths into the gain medium to amplify the optical signal, each wavelength of the optical signal being within a gain profile of the gain medium, the method further comprising: Clamping the gain of the gain medium over a spectral region including the wavelength of the optical signal using stimulated emission in the gain medium. The amplified optical signal is then extracted from the gain medium. In one preferred embodiment of the method, one or more co-dopants are provided to the gain medium to improve the non-uniform spread of the gain profile. In another preferred embodiment of this method, the gain can be controlled by changing the loss in the resonator. In yet another preferred embodiment of the method, gain flatness can be controlled by adjusting a wavelength dependent loss factor in the resonator.
[0017]
Detailed Description of the Preferred Embodiment
FIG. 3A shows one preferred embodiment of the present invention. The gain medium 20 is preferably an erbium doped fiber amplifier (EDFA) in which erbium acts as laser ions. The gain medium 20 forms part of the optical resonator 30. Doped integrated optical waveguides, other optical gain media such as bulk gain media, and semiconductors such as GaAs may also be used. The optical pump source 34 for pumping the erbium-doped gain medium 20 may advantageously include a near infrared diode laser that emits at one or more lines in the 950-1000 nm spectral region. Pump light is coupled to the erbium-doped fiber by a dichroic coupler 38, for example, a wavelength division multiplexer that couples all pump power to the resonator 30 but not the off-ring laser signal 54. FIG. 3A shows only one of a number of possible pump configurations. For example, by suitably placing one or more dichroic couplers 38 or combiners at either end of the erbium-doped fiber, the erbium-doped fiber 20 can be forward, backward, or simultaneously in both directions (bidirectional pumping). ) Can be pumped. The combiner may be a standard fiber wavelength division multiplexer, polarization beam combiner, or any number of waveguide or bulk optical combiners, well known in the art. The spectral output of the optical pump source 34 takes advantage of the non-uniform broadening inherent in erbium-doped fibers, taking advantage of the gain profile in either a single spectral region or a series of smaller, more closely spaced spectral regions. Selected to allow clamping of all or a substantial portion of the gain. For example, the optical pump source 34 can be operated at discrete wavelengths other than the absorption peak (wing) of the absorption transition, either on the long wavelength side or the short wavelength side of the erbium absorption transition. When the gain medium is an Er-doped fiber, a possible pump transition is around 1480 nm.^FourI_15/2→^FourI_13/2Transition and around 980nm^FourI_15/2→^FourI_11/2Transitions. However,^FourI_15/2→^FourI_13/2When pumping at the transition, it is not possible to pump outside of the long wavelength peak. Alternatively, both the absorption transition other than the short wavelength peak and the long wavelength peak may be pumped. Either a broadband pump source or a multiple wavelength source can be used. By pumping the gain medium 20 in this manner, the gain profile of the erbium-doped fiber behaves more non-uniformly than that pumped near the line center, thereby facilitating gain clamping over a wider area. The optical pump source 34 is preferably coupled into the resonator 30 by a dichroic coupler 38. As used herein, a broadband pump source is a broad spectral region (ie, a measurable variable width, such as the pump band used, such as a superfluorescent fiber source (SFS) or a source based on amplified spontaneous emission). It means a light source that emits light over a spectral region (with a line width of 20%). For example, the erbium-doped fiber 20 may be co-doped with ytterbium, which can be F. Wysokey, P.I. Namkuyo, D.C. PF Wysocki, P. Namkyoo, D. DiGiovanni “Dual-stage erbium / ytterbium co-doped fiber amplifier with output power up to +26 dBm and a flat spectrum of 17 nm (" Dual-stage erbium-doped " , erbium / ytterbium-codoped fiber amplifier with up to + 26dBm output power and a 17-nm flat spectrum ")" Optics Letters, Vol.21, no.21, pp.1744-1746, November 1, 1996, Be taught. As is well known in the art, such Er / Yb fibers can be pumped around 1060 nm, and pump radiation is absorbed by Yb ions in the amplifier fiber, which converts the excited energy into erbium ions. Transfers and results in an inversion distribution of erbium ions. Such an Er-Yb doped amplifier fiber can be pumped by a Yb doped superfluorescent fiber source (where ytterbium acts as a laser ion in the superfluorescent fiber source), where the source is a 1040-1080 nm window. It can be designed to emit high power over a wide spectral region in the vicinity. (See, for example, L. Goldberg, JP Koplow, RP Moeller, DAV Kliner, “Side-pumped Yb-doped doubles, L. Goldberg, JP Koplow, RP Moeller, DAV Kliner.) "High-power superfluorescent source with a side-pumped Yb-doped double-cladding fiber" "Optics Letters, vol. 23, no. 13, pp.1037-1039, July 1998 See Month 1) The bandwidth of the broadband pump source may be adjusted to the desired value, for example, by an internal or external filter, or other optical means.
[0018]
The resonator 30 is preferably a ring resonator in which the laser emission from the erbium doped fiber 20 is circulated unidirectionally within the resonator by the optical isolator 42, ie in the direction indicated by arrow 46. In order to control the losses in the resonator, preferably at least one attenuator 50 is used in the resonator 30. At a particular laser wavelength, the round trip loss in the laser resonator 30 is equal to the round trip gain, so the attenuator 50 also effectively controls the gain of the entire resonator. The attenuator 50 is advantageously variable (i.e. having a variable loss), or that loss produces a desired gain profile (e.g., flattening the gain profile) or depending on the wavelength. May be variable and wavelength dependent. For example, by introducing a non-uniform loss element in the gain profile to compensate for the loss spectrum in the resonator 30 that would otherwise be non-uniform and to produce a substantially flat gain spectrum. Is possible. Further, the wavelength dependent attenuator 50 may be located outside the resonator 30 instead of or in addition to the attenuator 50 inside the resonator. Several models of variable attenuators are commercially available, such as manufactured by the Johanson company of Boonton, NJ (eg, model number 2504F7B50C). The attenuator 50 is a photosensitive fiber grating (for example, AM Wengsaker et al. “Gain Equalizer Based on Long Period Fiber Grating” Opt. Lett., Vol. 21, pp. 336-338, March 1996). See also) or may include wavelength dependent loss elements such as mechanical fiber gratings.
[0019]
The input optical signal 54 is coupled to the optical resonator 30 via an optical isolator 58 and a first coupling such as an optical coupler 61 (eg, a coupler having 10% coupling (or 90% transmission) at the signal and laser wavelengths). Enters the device so that the input signal propagates in the direction of the erbium doped laser emission, ie the input signal 54 propagates in the direction indicated by arrow 66. After passing through the gain medium 20 and the dichroic coupler 38, the optical signal passes through a second coupling device, such as an optical coupler 62 (eg, also a 10% coupler) at port 63, and then the second isolator 70. Through the resonator 30, where the optical signal is referred to as the output optical signal 74. Since the ring laser emission circulates in a direction opposite to the direction of the signal 54 to be amplified, the ring laser signal is not output at the coupler 62 but is output at another port of this coupler, that is, the port 64. Thus, the embodiment of FIG. 3A allows the output optical signal 74 to be clearly separated from the laser emission of the erbium fiber 20.
[0020]
The couplers 61, 62 preferably have as small a coupling ratio as possible at the signal wavelength in order to minimize the loss given to the input signal 54. This means approaching the 0% coupler limit. For example, for some 1% couplers, the coupling “loss” received by the input signal 54 at coupler 61 (and the tap signal 74 at coupler 62) is very low (1%), which is preferred. Similarly, the coupling “loss” to the ring laser signal is very high (99%), which is also desirable because high cavity losses are desirable (to obtain high EDFA gain). Thus, couplers 61 and 62 are used to adjust the ring loss and hence the gain that the signal obtains (however, the relationship between the coupling ratio of the coupler and the net gain that the signal obtains must be carefully modeled). Thus, an alternative to using variable attenuator 50 is to change the gain level using the coupling ratio of either or both of the couplers.
[0021]
Referring to FIG. 3A, the lower the coupling ratio of the coupler 61, the lower the loss given to the input signal 54 by the coupler 61. Similarly, the lower the coupling ratio of the coupler 62, the lower the loss given to the signal amplified by the coupler 62. Thus, the lower the coupling ratio of couplers 61 and 62, the lower the loss experienced while the signal travels through the amplifier of FIG. 3A, and thus the higher the net gain that the signal receives (or vice versa) Lower pump power required to achieve net gain). In view of the above, it is advantageous to reduce both the coupling ratios for a given required net gain. One way to reduce the coupling ratio is to reduce the loss of other elements in the loop, specifically the attenuator 50 and the isolator 42. (In addition, the loss of the dichroic coupler 38 should be as low as possible. This has three benefits: the pump power loss in the dichroic coupler 38 is reduced; the signal power lost in the dichroic coupler 38. The amount is reduced; lower coupling ratios for couplers 61 and 62 may be selected). For example, if a clamped gain of 20 dB is required, one possible configuration is a 2 dB basic (wavelength independent) loss and a coupling ratio of 12.6% for each of couplers 61 and 62 (9 dB Transmission), ie, a 2 × 9 + 2 = 20 dB total loop loss (assuming that everything else in the loop element has negligible loss). is there. The preferred solution had 0 dB of background (wavelength independent) loss and 10% coupling ratio (or 10 dB transmission) for each of couplers 61 and 62, ie 2 × 10 + 0 = 20 dB overall loop loss. The wavelength-dependent attenuator 50 is used. In the former case, each of the two couplers 61 and 62 gives a loss of 12.6% to the signal. In the second case, each of the two couplers 61 and 62 gives 10% loss to the signal, which corresponds to a round trip signal loss of 2 dB less than in the first case.
[0022]
Alternatively, one (or both) of couplers 61 and 62 may be replaced with another coupling device such as an optical circulator. This is shown in FIG. 3B, except that couplers 61 and 62 have been replaced with optical circulators 81 and 82, and input and output isolators 58 and 70 have been removed. Similar to A). In this embodiment, the input signal 54 is not subject to separation loss in the input circulator 81 and the output signal 74 is not subject to separation loss in the output circulator 82. The benefit is that the signal loss is lower and thus amplifier 20 requires lower gain (and therefore requires lower pump power). However, unlike couplers 61 and 62, circulators 81 and 82 cannot provide the required high resonator loss (in the case of high gain amplifiers), and variable attenuator 50 must be used for this purpose. . Current commercial circulators exhibit small internal losses around 1 dB or just below 1 dB. However, there is no fundamental limit to this loss and it can be expected to be smaller in future circulator designs.
[0023]
The optical circulator 81 (and the optical circulator 82) is a three-port device, and is a well-known mode in which substantially all light entering through the port 84 is combined and output from the next adjacent port, ie, the port 83. Note that it works with: The optical circulator is a unidirectional device, which means that light circulates only in one direction in the circulator (counterclockwise in FIG. 3B). Thus, light returning from the resonator ring and entering from the port 83 of the circulator 81 is coupled through the third port 85 of the circulator 81, but is not output from the port 84 of the circulator 81. The circulator 81 thus operates as an isolator that prevents light entering the ring resonator from the port 84 from propagating directly to the port 85. An exemplary optical circulator is available from E-TEK Dynamics, Inc., Randy Avenue 1885, San Jose, CA 95131.
[0024]
Another advantage of the embodiment shown in FIG. 3B is that the input and output isolators 58 and 70 of FIG. 3A are no longer required. This is because the isolator 42 in the ring works together with the circulators 81 and 82 as an isolator. As far as output isolation is concerned, any floating signal returning from the output port 83 (of the circulator 81) to the output circulator 82 is directed to the isolator 42 where it effectively disappears without entering the Er-doped fiber amplifier 20. Thus, amplifier 20 is isolated from any feedback from output port 83. For input separation, no light exits the ring and enters the input port 84 (of the circulator 81). This is because any reverse signal from the Er-doped fiber 20 toward the input circulator 81 (especially the ASE signal generated by the Er-doped fiber and spurious reflections of the signal) enters the input circulator, which directs them to the ring. Because. Thus, this reverse signal does not enter the input port 84. This does not apply to the embodiment of FIG. 3A, but this embodiment requires an input isolator 58 since the input coupler 61 now directs 90% of the spurious signal to the input port 88 of the coupler 61. . Note that input and output isolators can still be used in the embodiment of FIG. 3B if a better isolator than that provided by circulators 81 and 82 is provided and isolator 42 is required. Instead of this, only one of the circulators may be used. For example, the circulator 82 may be replaced with the configuration of the coupler 62 / isolator 70 shown on the signal output side in FIG. 3A on the signal output side in FIG.
[0025]
The wavelength of the input signal 54 is preferably selected to fall within the gain profile of the gain medium 20, which is broad for erbium-doped fibers and is preferably at least 5 nanometers (nm) wide. When the pump power is high enough that the gain exceeds the loss, the resonator 30 receives an equal gain as all input signals pass through the erbium-doped fiber 20 by effectively clamping the gain across the gain profile. The embodiments of FIGS. 3A and 3B also provide pump power in a wide range (ie, the range between the pump power threshold and the highest pump power available from the pump power source). Provides the advantage of producing a gain that is insensitive to pump power changes over time.
[0026]
This is unresponsive because if the ring laser 30 is pumped above a relatively high threshold, the gain is significantly reduced from its small signal value by the circulating ring laser emission. If the pump power is increased above its nominal value, the gain will continue to be clamped to the same value, but the gain bandwidth will increase, as already described with respect to FIGS. 2 (C) and 2 (D). (Assuming that the gain bandwidth has not reached the optimum value of this nominal pump power). If the pump power is reduced from its nominal value, the gain will continue to be clamped to the same value (if the pump power is not reduced below the threshold) and the gain bandwidth will also decrease. . Therefore, (1) if the pump power does not drop below the threshold, the gain value does not respond to changes in pump power, and (2) the gain bandwidth depends on the pump power. However, for the lowest expected pump power, ensuring that the gain bandwidth is greater than the spectral bandwidth occupied by the combined input signal, the gain bandwidth is always wide enough and all Input signal 54 receives the same gain independent of pump power changes.
[0027]
Similarly, the embodiments of FIGS. 3A and 3B are non-responsive over some range of signal power and some input signal variations over some range of changes in some input signals. Provides the advantage of producing a gain that is insensitive to. This behavior can be explained as follows. If some input signal 54 is kept constant, but the power of some or all of the input signal is increased, the population inversion of the erbium-doped fiber remains constant, thereby keeping the gain constant. The laser accomplishes this by reducing the ring laser power. However, if the pump power is high enough, the laser will continue to fire, albeit over a narrower linewidth. Thus, the gain bandwidth is reduced, but the gain remains clamped to its original value. As explained in the previous paragraph, this decrease in gain bandwidth is still wide enough to provide a flat gain for all input signals 54 at its minimum possible value. It's a trivial thing. A similar argument is possible if several input signals 54 change while the individual signal power is kept constant. For example, if one or more of the input signals 54 drop, the gain remains clamped to the same value, but the gain line width increases (again, the maximum possible gain has not been reached). Suppose).
[0028]
This responsiveness to pump power, signal power, and the number of input signals 54 is particularly important in optical communication systems. For example, the number of input signals that pass through an amplifier, such as that described in this invention, may vary as the number of users varies over time or due to an accidental failure of one of the light sources providing the optical signal. The case may change. Similarly, input signal power and pump power can change over time, for example as a result of aging or failure of the light source supplying them.
[0029]
Although multiple wavelength pumping may be used, broadband pumping is expected to give better results. One broadband pump source for erbium-doped fiber 20 is a superfluorescent fiber source (SFS) made by ytterbium-doped fiber pumped around 980 nm, which is over several tens of mW in the 0.97-1.04 μm range. Fluorescence emission can be generated. (For example, DC Hanna, IR Perry, PJ Suni, JE Townsend, AC Tropper, DC Hanna, IR Perry, PJ Sani, JE Townsend, and AC Trapper) “Efficient superfluorescent emission at 974 nm and 1040 nm from an Yb-doped finer” from Opt. Comm, Vol. 72, nos. 3-4, pp .230-234, July 1989.) The spectral output of these fibers depends in part on their length, with longer fibers emitting at longer wavelengths. One short fiber (0.5 m) produces a 2 nm bandwidth emission at 974 nm, while a long fiber (5 m) produces a 19 nm bandwidth emission at 1040 nm. Such SFS is used in its short wavelength range for broadband pumping of Er-doped fibers (assuming that SFS is long enough to operate at 980 nm and its line width is sufficiently wide). SFS is also used in its long wavelength range for broadband pumping of Er / Yb doped fibers (typically pumped in the 0.98-1.064 μm range).
[0030]
Another embodiment of the invention relates to the composition of the core of erbium-doped fiber 20. Increasing the number of co-dopant species in the core results in more diverse physical locations where erbium ions can exist. Since each location induces a slightly different Stark separation of erbium ions, the non-uniform spread of erbium ions increases as co-dopant species is added. In general, the gain is expected to be more non-uniform as the number of network tuning co-dopants increases. The principle is (Er³⁺It applies to any laser ion (not just silica or fluoride glass) and any fiber host.
[0031]
A co-dopant that is preferably introduced into the core of the fiber 20 is a so-called network regulator, which tends to improve the solubility of rare earth ions in the glass host. Co-dopants that act at least in part as network regulators include, but are not limited to, K, Ca, Na, Li, and Al. Co-dopants known as index regulators such as Ge generally do not improve the solubility of rare earth ions, but can be introduced into the fiber, for example, to control the refractive index of the fiber. However, Ge tends to increase the non-uniform linewidth of the gain of erbium-doped silica-based fibers (see VL da Silva et al. Cited above).
[0032]
Improvements in non-uniform gain spread due to pumping other than at the peak of the absorption profile are demonstrated using the fiber ring laser 100 shown in FIG. Laser 100 includes a 3 meter long Er-doped fiber 104, two WDM fiber couplers 108 and 110, and an optical isolator 112 for directing laser resonance in a single direction. The fiber ring laser 100 is pumped by two pump laser diodes 116 and 118 operating at 980 nm. Laser diodes 116 and 118 are coupled into ring laser 100 via respective first and second WDM fiber couplers 108 and 110. The second WDM coupler 110 is also used to extract a laser signal from the ring laser 100. A third WDM coupler 120 is placed at the output of the ring laser 100 and is used to separate an unabsorbed 980 nm pump from the laser signal of the ring laser 100, the laser signal being in the range of about 1530 nm to about 1570 nm. The spectrum of each of these two signals is observed independently in the optical spectrum analyzer 130.
[0033]
FIG. 5 shows the spectrum of the output of the fiber ring laser 100 measured under two different pumping conditions. The ring laser 100 is at 978 nm with only one laser diode, ie an Er-doped fiber.^FourI_15/2→^FourI_11/2When pumped near the center of the absorption transition, the ring laser output exhibits a relatively narrow spectrum (tens of nm) centered around 1560.8 nm. On the other hand, if the ring laser 100 is pumped with two laser diodes, one at 974 nm and the other at 985 nm, the spectrum of the ring laser increases significantly, extending from about 1561 nm to 1563 nm. Other experiments suggest that a bandwidth of 14 nm or greater is achieved. Although the gain spectrum of the Er-doped fiber has not been measured with either pumping configuration, the results in FIG. 5 show that pumping the Er-doped fiber outside its absorption band peak is more than pumping only at the center of absorption. A broad emission is generated from the fiber, but this is more in the case of pumping other than the peak.³⁺It is assumed that the subset is due to simultaneous excitation. In addition to pumping other than the peak of the 980 nm absorption band disclosed herein, non-uniform broadening due to pumping of other than the peak of the 1480 nm absorption band will also be observed.
[0034]
The present invention may be embodied in other specific forms without departing from its spirit or principal characteristics. The described embodiments are to be considered in any sense illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.
[Brief description of the drawings]
1 (A), (B), (C) show how the frequency depends on the frequency when the pump power is below the laser emission threshold, above the threshold, and above for uniform spread. It is a figure which shows whether a gain changes.
FIG. 2 (A), (B), (C), (D) show that pump power is below the laser emission threshold, above the threshold, above, and notably with respect to non-uniform spread. It is a figure which shows how a gain changes with frequency when it is more than that, respectively. In (C), lasing occurs over a relatively narrow spectral range, whereas in (D), lasing occurs over a relatively wide spectral range.
FIGS. 3A and 3B are diagrams illustrating a preferred embodiment of the present invention in which an input signal is injected into an optical amplifier that produces a flat clamped gain across the gain profile of the gain medium. FIGS. is there.
FIG. 4 shows an experimental test setup for analyzing the spectrum output from an erbium-doped fiber placed in a ring laser.
FIG. 5 is a diagram showing how the spectrum output from the setup of FIG. 4 changes as a function of pump spectrum.

Claims (32)

An optical amplifier,
A ring optical resonator (30) for generating a clamped gain, the resonator (30) including a gain medium (20) having an absorption profile and a gain profile, wherein the gain profile is at least partially The gain medium is further pumped at both the non- short wavelength peak and the non- long wavelength peak of the absorption transition of the gain medium to flatten the non-uniform spread. An optical amplifier comprising an optical pump source (34) used for   The amplifier of claim 1, wherein the gain across the gain profile is substantially constant.   The amplifier of claim 1, wherein the gain medium (20) comprises a doped fiber.   The amplifier of claim 1, wherein the gain medium (20) comprises erbium, the erbium acting as laser ions.   The amplifier of claim 1, wherein the optical pump source (34) has an output that is broadband.   The amplifier of claim 1, wherein the optical pump source (34) has an output at at least one discrete wavelength.   The amplifier of claim 1, wherein the unidirectional ring resonator (30) includes an optical isolator (42) for regulating laser resonance in a unidirectional manner. The amplifier according to claim 7 , wherein the input signal (54) to the optical amplifier propagates in a direction opposite to the lasing direction (46).   The amplifier of claim 1, further comprising a wavelength dependent loss element (50) for compensating the non-uniform loss profile to produce a desired gain profile.   The amplifier of claim 1, wherein the optical pump source (34) is a superfluorescent fiber source. The amplifier of claim 10 , wherein the superfluorescent fiber source comprises ytterbium as laser ions.   The amplifier of claim 1, wherein the gain medium (20) comprises at least one co-dopant to improve non-uniformity of the gain medium. The amplifier of claim 12 , wherein the co-dopant includes a network regulator. The amplifier of claim 13 , wherein the co-dopant includes at least one element selected from the group consisting of K, Ca, Na, Li, Al, and Ge.   The amplifier of claim 1, wherein the gain medium (20) is flat over a spectral range of at least 5 nanometers and provides a wide gain.   The amplifier of claim 1, further comprising a variable attenuator (50) for controlling a level of loss to produce a desired level of gain in the gain medium.   The amplifier of claim 1, further comprising an input signal source for generating input signals (54) of different wavelengths. 18. An amplifier according to claim 17 , wherein the input signal (54) and lasing from the gain medium (20) propagate in opposition.   The amplifier according to claim 1, further comprising a coupling device (61, 62) for incorporating and extracting signals into and from the optical resonator (30). The amplifier according to claim 19 , wherein the coupling device (61, 62) comprises at least one optical coupler. The coupling device (61, 62) includes at least one optical circulator. The optical amplifier according to claim 19 . An optical amplifier,
An optical resonator (30) for generating a clamped gain, the resonator (30) including a gain medium (20) having an absorption profile and a gain profile, wherein the gain profile is at least partially Characterized by a uniform spread,
An optical pump source (20) that pumps the gain medium (20) at both non- short wavelength peaks and non- long wavelength peaks of the absorption transition of the gain medium and uses non-uniform broadening to change the gain ( 34)
An optical amplifier comprising a wavelength dependent loss element (50) for adjusting the loss to produce a desired gain profile. 23. The amplifier of claim 22 , wherein the optical resonator (30) includes the wavelength dependent loss element (50). 23. The amplifier of claim 22 , wherein the wavelength dependent loss element (50) adjusts the loss of the resonator (30) to produce a substantially flat gain. A method for producing an optical amplifier having a relatively flat gain, comprising:
Introducing a pump signal (54) into a gain medium (20) having an absorption profile and a gain profile, the gain medium (20) being in an optical unidirectional ring resonator (30), wherein the gain medium Is characterized by at least some non-uniform broadening, and the spectral output of the pump signal pumps both the non- short wavelength peak and the non- long wavelength peak of the absorption profile to deplete the gain medium (20). Selected to take advantage of uniform spread, and
Injecting a plurality of optical signals of different wavelengths into the gain medium (20) to amplify the optical signal (54), each wavelength of the optical signal (54) being a gain profile of the gain medium (20) Inside,
Clamping the gain of the gain medium (20) across a spectral region including the wavelength of the optical signal (54) using stimulated emission in the gain medium (20);
Extracting the amplified optical signal (74) from the gain medium (20). 26. The method of claim 25 , further comprising adding a co-dopant to the gain medium (20) to improve non-uniform spread of the gain profile. 26. The method of claim 25 , further comprising controlling a gain by changing a loss in the resonator (30). 26. The method according to claim 25 , comprising regulating the lasing in a direction (46) opposite to the direction of propagation (66) of the injected optical signal (54). 26. The method of claim 25 , wherein the gain medium (20) comprises a doped fiber. 26. The method of claim 25 , further comprising controlling gain flatness by adjusting a wavelength dependent loss element (50) in the resonator (30). 26. The method of claim 25 , wherein the stimulated emission comprises lasing.   The amplifier of claim 1, wherein the resonator (30) comprises a unidirectional ring resonator.

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